Field of the Invention
The present invention relates to a wireless communication system that uses white space, and more particularly, to a wireless communication system that is capable of efficiently making secondary use of a frequency without interfering with primary users.
Description of the Related Art
Description of Prior Art
With the remarkable progress of information society in recent years, wireless communication is, in addition to wired communication, being used more and more often as a communication method for many information communication appliances and services. This has resulted in ever-increasing demand for radio frequencies, which are limited resources, and exhaustion of frequencies that can be allocated is becoming a problem around the world.
Generally, frequency license management is performed by the state, and only a licensee is allowed to use a frequency under strict management, at specific time and location. However, to cope with the demand for frequencies which is expected to continue growing, a new method, free from the past method, of using frequencies is wanted.
Accordingly, in recent years, as a new method of using frequencies for solving the problem of exhaustion of frequencies, a method of using frequencies that are already allocated but are spatially and temporally not used (white space) is being studied.
For example, a cognitive wireless communication system or the like in which a user without a license (hereinafter referred to as a “secondary user”) flexibly uses radio waves in white space while sufficiently avoiding influencing use of frequencies by an existing system of a user with a license (hereinafter referred to as a “primary user”) is being studied and developed (for example, Non-Patent Literature 1).
As the cognitive wireless communication system, there is a wireless regional area network (WRAN) that uses white space and that is standardized by IEEE 802.22, for example.
Such a system includes, on an IP network, a white space database (DB) and a coexistence manager (CM) (hereinafter referred to as “database or the like”) for managing use of frequencies, and each wireless station acquires, by accessing the database or the like, a list of transmittable frequencies that is based on the position information of itself and the maximum allowed transmission power.
The list of transmittable frequencies is a list of frequencies at which wireless communication may be performed without interfering with primary users.
The list of transmittable frequencies is collectively managed while being updated as necessary by a spectrum manager (SM) of a base station (BS) to which a terminal (CPE: Customer Premises Equipment) such a slave station installed at a house or a mobile phone is connected.
Moreover, the BS determines the operating frequency of itself based on the list of transmittable frequencies.
Furthermore, each of the wireless stations (BS and CPE; the same applies in the following) may be provided with a spectrum sensing function.
When detecting by spectrum sensing that a determined use frequency is being used by an existing system (system of a primary user), the wireless station notifies the SM of the information. Then, the SM excludes the frequency from the list of transmittable frequencies, and notifies the database or the like to this effect.
A wireless communication system that uses white space avoids influencing the use of frequencies by primary users and also realizes communication by secondary users by performing dynamic spectrum access based on information that is updated from time to time in the above manner.
Incidentally, IEEE 802.22 does not clearly specify means used by a CPE not provided with the spectrum sensing function to grasp its transmittable frequency.
Now, generally, a CPE is not installed with a wireless interface other than a wireless interface for performing wireless communication with a BS (that is, an air interface of IEEE 802.22), and thus the CPE does not have means for connecting to an IP network (such as the Internet) other than a local area network without performing wireless communication with a BS.
Accordingly, when newly starting wireless communication that uses white space, a CPE has to emit radio waves to a BS before acquiring the list of transmittable frequencies that is provided by the database or the like based on the position information of the CPE.
In the case of a TDD (Time Division Duplex) system according to IEEE 802.22 or the like, the CPE and the BS have to perform wireless communication using the radio waves at the same frequency.
Therefore, a CPE about to newly perform wireless communication that uses white space performs, at the time of performing communication with a BS in order to grasp the transmittable frequency at the position of the CPE, transmission at a transmittable frequency of the BS.
Whether this frequency is a transmittable frequency for the CPE is unknown at this point, and there is a possibility that the frequency is not a transmittable frequency. Particularly, a CPE not provided with the spectrum sensing function cannot recognize use by a nearby primary user.
<Conventional Frequency Use Pattern: FIG. 5>
A frequency use pattern of a conventional wireless communication system will be described with reference to FIG. 5. FIG. 5 is a schematic explanatory diagram illustrating a frequency use pattern of a conventional wireless communication system.
In this case, CPEs 21 and 22 are present in a wireless communication system managed by a BS, and it is illustrated that the CPE 21 is already performing communication, and the CPE 22 is newly starting communication.
Furthermore, the BS is performing communication selecting f1 as the operating frequency from the list of transmittable frequencies presented to the spectrum manager, and the CPE 21 is already notified by the DB that f1 is the transmittable frequency.
Moreover, as illustrated in FIG. 5, with the conventional wireless communication system, the BS performs communication using the operating frequency f1 at all times, and thus it performs communication using the operating frequency f1 with not only the CPE 21 but also with the CPE 22 with which communication is to be newly started.
Here, since the CPE 22 is yet to acquire the list of transmittable frequencies from the DB via the BS, whether use of f1 is allowed is not known. That is, the CPE 22 is performing communication using the frequency f1 which may not be usable.